The production of an oxygen sensor element for and a A/F sensor for instance to be used in an exhaust gas stream, has heretofore been accomplished by a process involving the following steps of:
(1) forming a thin platinum layer for an oxygen detection electrode by electroless plating on the exterior of a zirconia ceramic formed in a cup or thimble-shape; PA1 (2) enhancing the denseness of the detection electrode by applying a heat; PA1 (3) forming a ceramic protective layer on the entire surface of the detection electrode for protecting the detection electrode; PA1 (4) forming another thin platinum layer for a reference electrode by electroless plating in the interior of the cup-shaped zirconia ceramic; and PA1 (5) aging the element in an atmosphere of a high temperature exhaust gas where the sensor is used.
The thin platinum layers for the detection and reference electrodes are formed by electroless plating. In this electroless plating, a solution of tetravalent platinum having a six-coordinate octahedral structure represented by hexaammine platinum (IV) tetrachloride ([Pt(NH.sub.3).sub.6 ]Cl.sub.4 ]) has been used. This complex has been reduced by a reducing agent to allow platinum to be deposited to a small thickness.
However, the inventors of the present invention think that since the hexaammine platinum (IV) tetrachloride has ligands as many as six and a stereostructure of octahedron, the ammine can be easily replaced by other ligands (e.g., chlorine) becoming impurities having other polygonal structures, making it difficult to remove the impurities and obtain a high purity product. If such impurities are present, their effect changes a metal deposition rate in the electroless plating, giving a tendency that the thickness of the thin platinum metal layer thus obtained can not be uniform i.e. partly great or small in the layer thickness. Further, when the production is subjected to a mass production basis, there appears a tendency that the layer thickness varies from lot to lot. In particular, the oxygen sensor elements formed in the cup-shape (or one-end closed cylinder shape) would show that the thickness of the thin platinum layer particularly inside the cup--cup shaped element, varies from an open end portion through a central portion to the closed end portion exhibiting different sensor properties and making it difficult to mass-produce sensor elements with a stable and high quality.
It is thought by the inventors of the present inventions that tetravalent platinum follows a reaction path involving the change of the valence of platinum (4.fwdarw.2.fwdarw.0) until it is deposited. It is likely therefor that the impurities would be produced when the tetravalent platinum changes its valence from 4 to 2. Thus, even if the tetravalent platinum has a high purity, the impurities can be present at the reaction process, making it difficult to mass-produce the sensor elements with the stable and high quality.
The present invention has been worked out to solve the foregoing problems found by the inventors.